The present invention is directed to a micro-electro-mechanical switching (MEMS) device having a beam that is actuated in a manner similar to an electrical relay, and specifically, to such a MEMS device beam having structures, and constructed in a manner, to minimize the distortion of the beam when subject to thermal expansion.
Due to the small size of a MEMS device and the materials from which it is made, the parts of the device are subject to closer tolerances and experience the effects of the environment much more greatly than larger devices. The MEMS device is made, preferably, from gold because of its electrical conducting properties and silicon for suitability for integrated circuit fabrication. The gold and silicon have different properties and are affected by the environment in different ways. In particular, when a MEMS device is manufactured and operated, it is subject to a variety of environmental conditions, such as excessive heat. When subject to excessive heat, the gold and silicon from which a MEMS device are made expand at different rates, which can cause distortion in the structure of the MEMS device. This material expansion and resulting distortion can be compensated for during the design process but only to a certain degree.
For example, the linear expansion of a material ΔL due to temperature can be determined fromΔL=α·L0·ΔT where α is the coefficient of thermal expansion, L0 is the length at the initial temperature, and ΔT is the change in temperature.
A configuration of a conventional MEMS switch is shown in FIG. 1 in a cross-sectional view. The MEMS switch 100 comprises a substrate 110 and a switch 120. The substrate 110 is formed from a semiconductor material such as silicon and coated with a dielectric material such as silicon dioxide or the like. The substrate 110 can also be a dielectric material such as sapphire or the like. The switch 120 includes an anchor 121, a hinge 123, a beam 125, and a tip 127. The anchor 121 couples the switch 120 to the substrate 110. The switch 120 also forms a current path or trace comprising the anchor 121, the hinge 123, the beam member 125, and the tip 127. The switch 120 is formed from gold, or some other suitable conductor. The current path via the anchor 121 is electrically connected to a source connection 113. The switch 120 is actuated by a voltage applied to a gate connection 115. The hinge 123 flexes in response to the voltage differential established between the gate connection 115 and the beam member 125 by the applied voltage. In response to the flexing, the tip 127 contacts the drain connection 117, completing a current path from a source connection 113 to the drain 117. Of course, the source connection 113 and the drain connection 117 can be interchanged without substantially affecting the operation of the MEMS switch 110. As the electric field at the gate connection 115 dissipates, the beam 125 raises thereby lifting the tip 127 from the drain connection 117.
During manufacturing, the MEMS device 100 can be subjected to high heat, such as approximately 400° C., which may cause distortion of the components of the MEMS device 100. Also, in operation, the MEMS device 100 will begin to experience heat, or thermal effects, associated with the application of voltage at gate connection 115 and electrical current through the current path from source connection 113 to drain connection 117, as well as heat from other sources on the substrate 110 or nearby, or even from the environment. In some cases of distortion, the beam 125 will lower toward the gate 115 due to thermal expansion and the tip 127 will contact drain connection 117. Although described with respect to a single switch, it is understood that the MEMS device 100 can comprise more than one switch 120 on a substrate 100, and the above description should not be interpreted to be limited to a single switch. In cases of multiple MEMS devices configured adjacent to one another, the beam 125 will distort such that the deflection of the tip 127 is different from that of neighboring tips. Such non-uniform deflection can be due to non-symmetric mechanical constraints, non-uniform fabrication process variations, other non-optimal operating conditions, other reasons, and/or combinations thereof. The non-uniform deflection may result, for example, in all tips 127 not making uniform contact, which can cause variations in the voltage required to actuate a particular switch in comparison to the voltage required to actuate other switches.
At the gold-substrate interface at an anchor point in a MEMS device, there is a difference in thermal expansions. Gold expands at almost 5 times the rate of silicon, and nearly 10 times the rate of silicon dioxide (SiO2). So there is a thermal expansion differential at differing points, such as the anchor point, of the MEMS device, with gold expanding the most. For example, at the gold-substrate interface, gold expands by approximately 0.56% when a temperature of 400° C. differential is applied, while silicon expands by approximately 0.12%. The difference in thermal expansion causes a shear force between them which can contribute further to distortion in the MEMS device and possibly device failure.
Because the substrate will not bend in the normal bimetallic fashion due to its much larger mass (i.e., the whole wafer), it is expected the gold at the interface would expand approximately 0.12%, although under stress, whereas the gold at the top of the MEMS device would expand at approximately 0.56%. This thermal expansion mismatch between the top and the bottom of anchor 121 is problematic because the distortion of anchor 121 may cause displacements in the beam 120 and the tip 127. If the thermal displacement at the tip 127 equals the separation distance between the tip 127 and the drain 117, the source 113 and the drain 117 will become electrically short-circuited. Further distortion at the anchor 121 will induce mechanical stresses in the beam 120 and the tip 127. Again, distortion caused by thermal expansion can cause performance problems in the MEMS device 100.
Another problem resulting from unmitigated thermal expansion is a tendency, in cases of multiple MEMS devices is for the beams 120 of MEMS devices to spread apart, in a shape similar to a hand-held fan, from one another in the horizontal plane. The spreading apart can cause misalignment of the components of the MEMS devices. Because of non-symmetric mechanical boundary conditions among the beams 120, such spreading apart will cause non-uniform tip displacements.
Accordingly, there is a need in the art to address the thermal expansion and distortion of the structures in the MEMS device, and thereby reduce complexity of associated circuitry that attempts to overcome the effects of the distortion.